In recent years, interest in developing renewable and “green” resources for chemical and fuel products has gained considerable momentum. In this respect, the exploitation of biomass or bio-based materials (i.e., materials whose carbon content is derived from regenerative biological rather than non-regenerative sources) for generating chemical and fuel products, which until now have been predominantly derived from fossil-origin materials, such as petroleum or coal, has become a focus of research and developmental investment. Certain chemical and fuel product replacements or alternatives have been produced on a commercial scale from biomass. For example, in the area of liquid fuels, ethanol and biodiesel (i.e., fatty acid alkyl esters) have been produced on a commodity scale from corn and sugar cane (for ethanol) and from various vegetable oils and animal fats. Even for these examples, though, biomass utilization processes can be improved.
Common raw materials derived from the processing of biomass are carbohydrates or sugars, which can be treated chemically to modify the carbohydrates into other useful chemicals. Thermal treatments provide a method to transform complex biomass, such as forest and agricultural residues into liquid oils. In a hydrothermal liquefaction (HTL) process, conversion of carbohydrates is done with wet biomass at elevated temperatures (e.g., 300°-350° C., 570°-660° F.). Steam generated by heating the wet biomass results in high pressures (e.g. 15-20 MPa, 2,200-3,000 psi). Typically, the conversion is processed in a matter of minutes (e.g., 5-20 minutes).
Another such chemical treatment process is to hydrogenate carbohydrates into polyhydric alcohols, which in turn can themselves be further processes into other useful materials or biofuels. Sugar alcohols, such as xylitol, sorbitol, and lactitol, are industrially most commonly prepared by catalytic hydrogenation of corresponding sugar aldehydes over sponge-metal catalysts, such as nickel and ruthenium on carbon catalysts.
Coking and catalyst deactivation is a problem that arises from the hydrogenation of sugar alcohols because of the presence of residual sugars and high molecular weight polymers that have a degree of polymerization (DP) number greater than 3 in sugar alcohol solutions.
In many cases process designs, costs and operation schedules are greatly affected by the presence of catalyst poisons. Presence of oxidizing agents or small amounts of deactivators can cause either deactivation or poisoning of the hydrogenation catalyst. Sometimes the reaction product, reaction intermediates or by products act as catalytic deactivators and do not allow completion of the primary reaction.
In large scale sugar hydrogenation, catalyst deactivation often plays a central role in the economic efficiency of the hydrogenation process, such as life cycle assessment (LCA). Deactivation of the catalyst can be a complex phenomenon because active sites on catalyst may be blocked by bulky molecules through physical absorption, or poisoned by impurity in the feed stream, or absorbance of reactants, intermediates and products. Among of the latter, catalyst poisoning by impurities, such as sulfur compounds, is a key factor for catalyst deactivation. Although, in general, the amount of sulfur contained in biomass is relatively small, however, at large volumes and over time even minimal amounts can build up and adversely affect catalyst activity. Some biomass can contain as much as 0.5 wt. % sulfur. This poisoning impurity has strong interaction with catalyst surface and can be irreversible.
In a particular situation, one of the problems encountered in the catalytic hydrogenation of aldose sugars is the deactivation and instability of the hydrogenation catalyst, for example due to the formation of harmful by-products, such as epimers, hydrolysis products and their reduction products. Aldonic acids, such as lactobionic acid and xylonic acid, represent one example of the harmful by-products formed in the hydrogenation of aldoses to alditols. In the hydrogenation of glucose, it has been found that gluconic acid is typically formed as a by-product. It has also been found that gluconic acid has a tendency to adhere to the catalyst surface thus occupying the active sites of the hydrogenation catalyst and deactivating the catalyst. The deactivation and instability of the catalyst also lead into problems in the recovery and regeneration of the catalyst. These problems are even more severe especially with recycled catalysts. Recovery of the catalyst by filtration can be difficult.
In view of the foregoing, care should be taken to minimize presence of catalytic deactivators and poisons in the reaction mixture so as to prolong life of the catalyst. Under such situations some may either opt to use a different type of catalyst or seek to clean or regenerate the catalyst.
Regeneration of deactivated catalysts is possible for many catalytic processes and is widely practiced. The main purpose is to remove the temporary poisons on the catalyst surface and restore the free adsorption sites. Generally regeneration processes can be categorized into two types, i.e., off-site and on-site regeneration. In the off-site (ex-situ) regeneration, the catalyst is unloaded from the reactor and regeneration is performed in moving-bed belt calciners or conical-shaped rotating drum calciners. (See, Robinson, D. W., Catalyst Regeneration, Metal Catalysts. Kirk-Othmer Encyclopedia of Chemical Technology [online]. 4 Dec. 2000). The on-site (in-situ) regeneration does not require removing the catalyst from a reactor. Commonly, the procedure is to burn off, or oxygenate, the temporary poisons, such as green oil, in order to resume catalyst activity. Regeneration of the catalyst may be accomplished, for example, by heating the catalyst in air to a temperature over 300° C., up to about 500° C., to incinerate any organic material, polymers, or char. Catalyst regeneration using such techniques, however, has certain limits; one is that repeated regeneration operations can cause permanent degradation of the catalyst activity.
Another approach is using hot-compressed water as alternatives to organic solvents and as a medium for unique and/or green chemistry to extract a variety of organic compounds has grown over the recent decade. (Adam G. Carr et al., A Review of Subcritical Water as a Solvent and Its Utilization for Processing of Hydrophobic Organic Compounds, CHEMICAL ENGINEERING JOURNAL, v. 172 (2011), pp. 1-17, contents of which are incorporated herein by reference. See also, e.g., M. Osada et al., ENERGY & FUELS 2008, 22, 845-849, contents incorporated herein by reference, pertaining to regenerate catalysts poisoned by sulfur.) Of particular interest are processes in water at or near its critical point (Tc=374° C., Pc=221 bar (˜3205.33 psi), and pc=0.314 g/ml). Although some have explored the use of subcritical water as a solvent and its utility for hydrophobic organic compounds, different kinds of reaction materials and catalytic substrates bring their own associated and distinguishable issues. The reversal of the solvent characteristics of critical hot-compressed water also results in precipitation of salts that are normally soluble in room temperature water. Most inorganic salts become sparingly soluble in supercritical water. This is the basis for unique separation of ionic species in supercritical water. The precipitated salts can serve as heterogeneous catalysts for reactions in supercritical water.
In view of the various problems and limitations current regenerative techniques, a better process for regenerating catalysts used in sugar hydrogenation would be appreciated.